Melt the butter over low heat in a large pot. Remove from heat and add the marshmallows. Stir constantly until the marshmallows are completely melted and combined with the butter.

Add the crispy cereal to the pot and stir well to completely coat in the marshmallow mixture.

Spray a 13"x9" pan with cooking spray then transfer crispy marshmallow mixture to the pan. Using wax paper, flatten the crispy mixture until it is even in the pan.

Let cool and then use heart-shaped cookie cutters and remove heart crispy treats from pan onto a plate.